1. Field of the Invention
This invention relates generally to a vehicle anti-theft system for preventing theft of a vehicle and, more particularly, to a vehicle anti-theft system that disables the vehicle's engine after the engine is running, if an unauthorized vehicle start-up has occurred, by disconnecting the vehicle battery and draining the alternator voltage to ground.
2. Discussion of the Related Art
Theft of vehicles is a very significant and important problem facing vehicle owners. To address this problem, many different types of anti-theft systems that attempt to prevent a potential thief from stealing a vehicle are known in the art. However, as the sophistication of the anti-theft systems increases, the sophistication and ingenuity of the thieves also increases. The different types of anti-theft systems include steering wheel locks, alarm systems, and devices that disable certain essential components of the vehicle ignition system when an attempted theft is recognized. The systems that disable essential components of the vehicle ignition system generally include systems that interrupt the vehicle ignition circuit so as to prevent the vehicle from actually starting. One particular system includes providing an authorization circuit within the vehicle ignition key that is electrically coupled to the ignition circuit when the key is put into the ignition switch to provide a signal of an authorized user to enable the vehicle to be started.
Some of the known anti-theft systems attempt to prevent theft of a vehicle by disconnecting the vehicle battery from the ignition circuit to prevent the vehicle from being started, when an unauthorized ignition is detected. For example, U.S. Pat. No. 4,958,084 issued to Carlo et al. discloses an anti-theft device that prevents the vehicle from being started by disconnecting the vehicle battery from the ignition circuit upon detection of a high current flow indicative of an attempt to start the vehicle. The Carlo anti-theft device includes a power switch having a first condition connecting the battery to the ignition circuit to allow the battery to deliver power to the ignition circuit and a second condition that disconnects the battery from the ignition circuit. The power switch is shifted from the first condition to the second condition in response to a flow of a substantial amount of current through the ignition circuit that shows an attempt to start the vehicle. An externally received coded command signal from a hand-held transmitter deactivates the shifting of the power switch to the second condition when the high current flow is detected to enable the authorized user to start the vehicle.
The Carlo anti-theft system suffers from a number of drawbacks that can be improved upon. For example, because the Carlo anti-theft system senses the large amount of current (about 800 amps) that is present during ignition just prior to starting the vehicle engine, high integrity and robust mechanical elements are necessary to disconnect the vehicle battery at that time. Therefore, the Carlo power switch must be a heavy-duty and expensive mechanical switch. Further, because the Carlo anti-theft system only disconnects the battery from the ignition circuit to prevent the engine from being started, the system is susceptible to jump starting to complete the theft, where a second battery is connected to the vehicle starting system. The second battery would not be disconnected from the starting circuit by the anti-theft system. Also, because modern vehicles usually start very quickly after the ignition key has been turned, the chance that the Carlo anti-theft system will be able to sense the high current in time to prevent the vehicle engine from starting in these vehicles is reduced.
What is needed is an anti-theft system that disables a vehicle engine by disconnecting the battery from the vehicle ignition system, but does not suffer from the drawbacks discussed above. It is therefore an object of the present invention to provide such an anti-theft system.